1. Field of the Invention
The present invention relates to a process for purifying a crude acrylic acid melt which contains, based on its weight, which does not include water present in the crude acrylic acid melt,                ≧80% by weight of acrylic acid and, as impurities different from acrylic acid, at least        ≧100 ppm by weight of acetic acid and        ≧10 ppm by weight of propionic acid,in which the crude acrylic acid melt is converted, under the action of low temperatures, into a crude acrylic acid suspension consisting of acrylic acid crystals and residual melt, the amount by weight of impurities different from acrylic acid in the acrylic acid crystals being less than, and the amount by weight of impurities different from acrylic acid in the residual melt being greater than, the amount by weight of impurities different from acrylic acid in the crude acrylic acid melt, if necessary a part of the residual melt is separated off mechanically from the crude acrylic acid suspension and the acrylic acid crystals of the remaining crude acrylic acid suspension are freed from remaining residual melt in a wash column.        
2. Description of the Background
Acrylic acid, either as such or in the form of its salts or its esters, is especially important for the preparation of polymers for a very wide range of applications (e.g. adhesives, superabsorbers, binders).
Acrylic acid is obtainable, inter alia, by catalytic gas-phase oxidation of propane, propene and/or acrolein. These starting gases, as a rule diluted with inert gases, such as nitrogen, CO2 and/or steam, are passed, in the form of a mixture with oxygen at elevated temperatures and, if required, superatmospheric pressure, over transition metal mixed oxide catalysts and are converted by oxidation into a product mixture containing acrylic acid. By condensing the product mixture or by taking up in a suitable absorbent (e.g. water or a mixture of from 70 to 75% by weight of diphenyl ether and from 25 to 30% by weight of biphenyl), basic separation of the acrylic acid from the product gas stream can be achieved (cf. for example EP-A 297 445 and German Patent 2 136 396).
By removal of the absorbent (and, if required, prior desorption of impurities having poor solubility in the absorbent, by stripping, for example, with air) by extractive and/or distillative separation processes (e.g. removal of the absorbent water by distillation, azeotropic distillation or extractive separation of the acid from the aqueous solution and subsequent removal of the extraction medium by distillation) and/or after the use of other separation steps, an acrylic acid is frequently obtained which is referred to here as crude acrylic acid.
This crude acrylic acid is not a pure product. Rather, it contains a range of different impurities typical for the gas-phase catalytic oxidative preparation route. These are in particular propionic acid, acetic acid and low molecular weight aldehydes (typically, the total content of low molecular weight aldehydes in crude acrylic acid to be treated according to the invention is ≧100 ppm by weight, based on the weight of crude acrylic acid calculated as anhydrous; as a rule, the abovementioned aldehyde content is ≦1% by weight), such as acrolein, methacrolein, propionaldehyde, n-butyraldehyde, benzaldehyde, furfurals and crotonaldehyde. Depending on the method of preparation of crude acrylic acid, it may also contain water as a further impurity. Another typical component of crude acrylic acids are polymerization inhibitors. These are added in the course of the separation processes used for the preparation of crude acrylic acid, where they are intended to suppress a possible free radical polymerization of the α,β-monoethylenically unsaturated acrylic acid, which is why they are also referred to as process stabilizers. Dibenzo-1,4-thiazine (PTZ), 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl(4-OH-TEMPO) and p-methoxyphenol (MEHQ) occupy an outstanding position among the acrylic acid process stabilizers and may be a component of crude acrylic acid either in each case by themselves or in pairs or as a mixture of three substances. Usually, the total amount of polymerization inhibitors contained in crude acrylic acid is from 0.1 to 2% by weight, based on the weight of the crude acrylic acid (the water present in the crude acrylic acid is not included).
Further undesired impurities of acrylic acid present in the condensed phase are the acrylic acid oligomers (Michael adducts) formed by Michael addition of acrylic acid with itself and with the acrylic acid dimers formed therein. While these compounds are usually virtually completely absent from freshly produced crude acrylic acid (usually, their amount by weight is <0.01% by weight, based on the weight of crude acrylic acid calculated as anhydrous), they form therein when said crude acrylic acid is left to stand for a long time. For statistical reasons, the formation of diacrylic acid is of particular importance, whereas the formation of higher acrylic acid oligomers (trimers, tetramers, etc.) takes place only in a minor amount.
The total amount of other byproducts possibly contained in crude acrylic acid is as a rule not more than 10% by weight, based on the weight of the crude acrylic acid calculated as anhydrous.
In this publication, crude acrylic acid is therefore to be understood as meaning in particular that crude acrylic acid which contains, based on its weight calculated as anhydrous,                ≧80% by weight of acrylic acid,        from ≧100 ppm by weight to ≦15% by weight of acetic acid,        from ≧10 ppm by weight to ≦5% by weight of propionic acid,        up to 5% by weight of low molecular weight aldehydes,        up to 3% by weight of polymerization inhibitors and from 0 to 5% by weight of acrylic acid oligomers (Michael adducts).        
The term crude acrylic acid used here thus also comprises in particular that crude acrylic acid which contains, based on its weight calculated as anhydrous,                ≧90% by weight of acrylic acid,        from ≧100 ppm by weight to ≦5% by weight of acetic acid,        from ≧10 ppm by weight to ≦2% by weight of propionic acid,        up to 2% by weight of low molecular weight aldehydes,        up to 2% by weight of polymerization inhibitors and        from 0 to 3% by weight of acrylic acid oligomers (Michael adducts).        
Not least, the term crude acrylic acid used here therefore comprises that crude acrylic acid which contains, based on its weight calculated as anhydrous,                ≧95% by weight of acrylic acid,        from ≧100 ppm by weight to ≦3% by weight of acetic acid,        from ≧10 ppm by weight to ≦2% by weight of propionic acid,        up to 2% by weight of low molecular weight aldehydes,        up to 2% by weight of polymerization inhibitors and        from 0 to 2% by weight of acrylic acid oligomers (Michael adducts).        
Based on the amount of acrylic acid contained in the crude acrylic acid, the crude acrylic acids frequently contain no water, but in some cases also up to 5% by weight or up to 4% by weight or up to 3% by weight of water.
Of the components, apart from acrylic acid, contained in the abovementioned crude acrylic acids, most prove to be disadvantageous during the use of crude acrylic acid.
If such a crude acrylic acid were used, for example, for the preparation of esters of C1-C8-alkanols and acrylic acid, the corresponding acetic esters and propionic esters would also be formed in secondary reactions, which reduces the yield of desired acrylic esters, based on the amount of alkanol used. If the acrylic esters formed in the presence of low molecular weight aldehydes are used for free radical polymerizations, their content of the low molecular weight aldehydes generally has a disadvantageous effect, for example, in that they influence, for example, the induction time of polymerization reactions, i.e. the period between reaching the polymerization temperature and the actual beginning of the polymerization. Furthermore, they generally influence the degree of polymerization and may also cause discolorations of the polymers.
The abovementioned disadvantages are usually also applicable when the crude acrylic acid is used directly as an acrylic acid source in polymerizations.
Acrylic acid sources to be used for the production of superabsorbers (=materials for absorbing water and based on polyacrylic acid and its salts) are subject in particular to the requirement that they may contain very little diacrylic acid and very little dibenzo-1,4-thiazine, since both components are undesirable either in the production of superabsorbers (in particular dibenzo-1,4-thiazin presents problems owing to its extremely inhibiting effect on free radical polymerizations in the production of superabsorbers) or in the use of the superabsorbers (superabsorbers are used in particular in the hygiene sector (e.g. baby's diapers); the final stage of superabsorber production consists as a rule of a high-temperature surface crosslinking; at the crosslinking temperatures used, copolymerized diacrylic acid will be at least partially cleaved (inverse Michael addition) with formation of monomeric acrylic acid; an increased content of monomeric acrylic acid is however not tolerable in this area of use).
In practice, separation operations involving rectification are used in particular for further purifying crude acrylic acid (cf. for example EP-A 722 926).
The disadvantage of these separation processes is that they require a large amount of energy, in particular for separating off the components having a similar boiling point to acrylic acid (e.g. propionic acid), since the procedure has to be carried out with high reflux ratios and/or with rectification columns having a large number of theoretical plates. Therefore attempts have also already been made, for example, to synthesize acrylic acid free from propionic and/or acetic acid by appropriate adaptation of the gas phase oxidation process (cf. e.g. JP-A 11 35519 and EP-A 253409). Moreover, the thermal stress generally causes undesirable free radical polymerization of the acrylic acid in the case of purification processes involving rectification.
As an alternative, the melt crystallization procedure has been increasingly used in the very recent past for the preparation of pure acrylic acid (cf. for example EP-A 616 998). Very generally, the contaminated crude acrylic acid (melt) is partially solidifed by cooling. Depending on the phase equilibrium, the corresponding acrylic acid crystals have a lower content of impurities than the remaining liquid residual melt. The separation effect described above and determined purely by thermodynamic factors is weakened by the inclusion of liquid during the crystallization process and by the residual melt still adhering to the solid after the solid/liquid separation. For achieving higher purities, a plurality of successive crystallization stages are often therefore necessary, even in the case of eutectic systems, i.e. the crystals obtained in a first crystallization stage are melted again and subjected to a further crystallization step, etc. The disadvantage of such a successive procedure is that, in each stage, the heat of crystallization has to be removed on freezing and supplied again on subsequent melting. This adversely affects the cost-efficiency of separation processes involving crystallization. For economical use of the melt crystallization process, it is therefore of very decisive importance to achieve very high purity of the isolated crystals with very few crystallization stages.
For the purification of crude acrylic acid melts by crystallization, the prior art predominantly recommends layer crystallization processes (cf. for example German Laid Open Application 2,606,364, EP-A 616 998, EP-A 648 520 and EP-A 776875).
In the layer crystallization processes the crystals are frozen in the form of cohesive, thermally adhering layers. The solid/liquid separation is effected by simply allowing the residual melt to flow away. The purified crystals are then melted. In principle, distinction is made between static and dynamic layer crystallization processes.
In the static processes, the crude acrylic acid melt to be purified is introduced, for example, into tube-bundle or modified plate-type heat exchangers and then partially solidifed by slowly lowering the temperature on the secondary side. After the freezing, the residual melt is discharged and then, by slowly increasing the temperature, first more highly contaminated and subsequently less contaminated fractions from the crystal layer are melted until finally the product having high purity is melted. This process is referred to in the literature as sweating. It is true that static crystallization processes achieve a significant purification effect in one crystallization stage in the case of crude acrylic acids. However, the usually low space-time yield in static crystallization processes is disadvantageous since, in static melt crystallization, the heat transport and mass transfer to the deposition surfaces takes place only by free convection.
Typical of the dynamic layer crystallization of crude acrylic acid melts is forced convection of the crude acrylic acid melt. This can be effected by circulating the crude acrylic acid melt by pumping through tubes with full cross-sectional flow (e.g. German Laid Open Application 2,606,364), by adding the crude acrylic acid melt as a trickle film (e.g. EP-A 616 998) or by passing inert gas into a tube filled with melt or by pulsation.
The disadvantage of a purification of crude acrylic acid melts by dynamic layer crystallization is that the purification effect within one crystallization stage with high impurity contents of the crude acrylic melt is unsatisfactory, and it is for this reason that EP-A 616 998 recommends the use of a combination of static and dynamic layer crystallization for purifying crude acrylic acid melts. However, the disadvantage of this procedure is that it necessarily requires a plurality of crystallization stages. A certain improvement can be achieved by using the washing, recommended in DE-A 3 708 709, of the deposited crystal layers with purer melt fractions. Owing to the small specific surface area of the deposited layers however, the wash effect is not completely satisfactory.
EP-A 616 998 includes the possibility of a suspension crystallization for the purification of crude acrylic acid melt by crystallization, but washing of the isolated suspension crystals to remove adhering residual melt is not considered. Instead, a combination with static crystallization stages is recommended, which is unsatisfactory owing to the inevitable multistage nature of the procedure.
In the suspension crystallization process, as a rule a crystal suspension which consists of crystals having a lower impurity content and a residual melt having a higher impurity content is produced by cooling the starting melt containing the impurities. The solid crystals may grow while present directly in suspension or may be deposited as a layer on a cooled wall, from which they are then scratched off and resuspended in the residual melt, i.e. the solids formation can be carried out in cooled stirred kettles, in scraped-surface heat exchangers or in disk crystallizers, as described, for example, in Chem.-Ing.-Techn. 57 (1985) No. 2, 91-102.
Subsequently required separation of the residual melt from the crystal can initially be carried out purely mechanically by pressing off, filtration and/or centrifuging (cf. for example Chem.-Ing.-Techn. 57 (1985) No. 2, 91-102).
The disadvantage of such a procedure with purely mechanical separation of crystals and residual melt is that, owing to the remaining residual melt adhering to the crystals, the purification effect achievable in one separation step is unsatisfactory in the case of crude acrylic acid melts.
The prior application DE-A 19926082 therefore recommends subsequently washing the mechanically removed acrylic acid suspension crystals additionally with a wash liquid containing acrylic acid, the wash liquid used preferably being an acrylic acid melt whose amount by weight of impurities different from acrylic acid is less than the corresponding impurity content of the mechanically isolated suspension crystals to be washed.
The disadvantage of the washing method used in DE-A 19926082 is that its purification effect is not completely satisfactory. This is presumably due in particular to the fact that the contact achieved between crystals to be washed and wash liquid is not completely satisfactory.
It is now generally known that, in the case of a slurry of suspension crystals, separation of suspension crystals and residual melt may also be carried out either exclusively, or after partial mechanical separation (in particular before use of a mechanical wash column) of residual melt, by means of a suitable washing liquid in a wash column in which the wash liquid is passed countercurrently, to the suspension crystals.
In principle, the wash column types are divided (cf. FIGS. 1 to 4) into those with forced transport of the suspension crystal bed and those with gravity transport of the suspension crystals (a detailed description of the different wash column types is to be found, inter alia, in Chem.-Ing.-Techn. 57(1985) No. 2, 91-102, in Chemical Engineering Science 50, 1995, No. 17, 2712 to 2729, Elsevier Science Ltd., in Applied Thermal Engineering 17, (1997) No. 8-10, 879-888, Published by Elsevier Science Ltd., and the citations stated in the abovementioned references). In wash columns with forced transport of the suspension crystal bed, at least one force other than gravitation in the transport direction is used for transporting the suspension crystal bed.
Inside the wash column, the suspension crystals are transported either from top to bottom or from bottom to top. The wash liquid is passed countercurrently to the suspension crystals in the wash column. In the prior publications DE-A 19626839, DE-A 19740252, DE-A 19829477, DE-A 19832962, DE-A 19833049 and DE-A 19838845 inter alia water or aqueous acrylic acid is recommended as wash liquid to be used for crude acrylic acid suspensions. However, the disadvantage of these wash liquids is that, on the one hand, the purification effect is not completed satisfactorily and, on the other hand, they result in considerable acrylic acid losses.
As an alternative to the abovementioned procedure, it is also possible to melt the suspension crystals reaching the wash column in purified form at the end of their transport distance (the mother liquor is removed as a rule in the opposite part of the wash column), to remove only a portion of the resulting purified melt and to recycle the remaining amount of the purified melt as wash melt to the wash column and to do so countercurrently to the suspension crystals fed to the wash column (in this publication, wash columns operated in this manner are to be referred to in the narrower sense as wash-melt wash columns). Depending on the physical characteristics of the crystal suspension to be treated in the wash column, a purification effect may be achieved either on the basis of all or only on the basis of some of the various mechanisms listed below:                Displacement of the residual melt (mother liquor) by the wash melt,        Washing away of the layer of residual melt adhering to the suspension crystals by the wash melt,        Diffusion washing of the few/no regions with throughflow between suspension crystals (for example those in contact over large areas) with wash melt,        Crystallization of the wash melt, recycled into the wash column, on the suspension crystals fed countercurrently,        Sweating of the suspension crystals in contact with the wash melt,        Adiabatic recrystallization of the suspension crystals in contact with the wash melt.        
The last three of the abovementioned purification mechanisms are to be referred to here as additional purification mechanisms.
According to Chemical Engineering Science 50, (1995) No. 17, 2717-2729, Elsevier Science Ltd., the contribution of the individual purification mechanisms depends, inter alia, on the time of contact between the suspension crystals and the wash melt and on the morphology and composition of the suspension crystals. In principle, none of the abovementioned purification mechanisms can be excluded because, owing to the smaller melting point depression by impurities, the melting point of the purified crystals is higher than the temperature of the still unwashed crystals, which essentially corresponds to the equilibrium temperature of the crude acrylic acid suspension.
In the case of gravity wash-melt wash columns, the suspension crystals are transported along the wash column by gravity countercurrently to the wash melt which has a lower density and hence a lower specific gravity (and therefore ascends in the wash column). A slowly rotating stirrer (usually <0.035 revolutions per second) frequently extends over the entire gravity wash-melt wash column and serves for preventing agglomeration and/or channel formation in the descending crystal bed. The residence time of the suspension crystals in a gravity wash-melt wash column is ≧1 hour (the difference between the density of the liquid phase and that of the solid phase is as a rule ≦15%). Furthermore, the lowest porosity within the crystal bed in a gravity wash-melt wash column is usually >0.45, often ≦0.65. The mother liquor leaves the gravity wash-melt wash column usually via an overflow. The advantage of a gravity wash-melt wash column is that its long residence times of the crystals utilize the additional purification mechanisms to a particular extent. According to Applied Thermal Engineering 17, No. 8-10, (1997), 879-888, Elsevier Science Ltd., a weak point of the gravity wash-melt wash column is a requirement for relatively large crystals.
In the case of wash-melt wash columns with forced transport of the suspension crystal bed, a distinction is made for example between pressure columns (also referred to as hydraulic columns), in which the crystals and the wash melt are transported, for example, externally by pumps and/or hydrostatic level and the mother liquor is generally forced out of the wash column via a filter (on the other side of the filters, atmospheric pressure, reduced pressure or superatmospheric pressure may prevail), and mechanical columns having mechanical force transport means for the crystal bed, such as special rams, stirrers, screws, helices or spirals. Mechanical wash-melt wash columns are particularly suitable for purifying crystal suspensions containing little residual melt. The mother liquor is removed in mechanical wash-melt wash columns, as a rule likewise via filters which are present either behind or in the mechanical forced transport means.
Wash-melt wash columns with forced transport of the crystal bed are distinguished by very much shorter residence times of the crystals in the wash column compared with the gravity wash-melt wash column. Said residence time is ≦30 minutes and is as a rule from 10 to 15 minutes, frequently from 2 to 8 minutes. Furthermore, the lowest porosity (=pore volume/total volume) within the crystal bed of a wash-melt wash column with forced transport is usually ≦0.45. According to Chemical Engineering Science 50, No. 17, (1995) 2717-2729, Elsevier Science Ltd., the residence times of the crystals in wash-melt wash columns with forced transport are too short for there to be a significant probability that the additional purification mechanisms will occur.
JP-A 7-82210 discloses a process for the purification of crude acrylic acid by crystallization, in which first a crude acrylic acid suspension is produced from the crude acrylic acid melt in the presence of water by the action of low temperatures, the purpose of the presence of water being to produce the required low temperatures by evaporative cooling. JP-A 7-82210 mentions only in passing that the presence of the water influences the formation of the acrylic acid crystals such that particularly large crystals grow.
In JP-A 7-82210, the crude acrylic acid suspension produced is finally subjected to purification treatment by means of a gravity wash-melt wash column. Although the purification effect achieved in JP-A 7-82210 with one purification stage is satisfactory, the space-time yield achieved is not. JP-A 7-82210 also notes that the presence of the water during the production of the crude acrylic acid suspension has an advantageous effect on the purity of the acrylic acid crystals washed in the gravity wash-melt wash column. The use of a gravity wash-melt wash column is also recommended in EP-A 730893.
For a satisfactory purification of crude acrylic acid by crystallization in one purification stage (in particular for the satisfactory removal of the impurities propionic acid and/or acetic acid) WO 99/06348 recommends first adding a polar organic substance to the crude acrylic acid, then producing an acrylic acid suspension with the action of low temperatures and washing said suspension in a mechanical wash-melt wash column.
The disadvantage of this procedure is that it requires the addition of a polar organic solvent to the crude acrylic acid.
From a paper by M. Nienoord, G. J. Arkenbout and D. Verdoes on Experiences with the TNO-Hydraulic Wash Column” at the 4th BIWIC 94/Bremen International Workshop for Industrial Crystallization, Bremen, Sep. 8th -9th, 1994, at the University of Bremen, Ed.: J. Ulrich, it is known that hydraulic wash-melt wash columns are suitable in principle for purifying acrylic acid suspensions. However, the abovementioned citation contains no information on the composition of the acrylic acid suspension or on its preparation.
In view of the abovementioned prior art, it is an object of the present invention to provide an improved process for the purification of crude acrylic acid melts which, on the one hand, is capable of providing acrylic acids of high purity and with a high space-time yield in only one purification stage and, on the other hand, requires no prior addition of a polar organic solvent to the crude acrylic acid, in particular for satisfactory removal of propionic and/or acetic acid.